1. Field of the Invention
The present invention relates to a high-frequency power amplifying circuit which is capable of optionally varying the power supplying mode.
2. Description of the Related Art
Either the "class A" amplification mode or the "class AB" amplification mode composing the intermediate amplification mode between the "class A" amplification and the "class B" amplification is made available for the output stage of the power amplifying circuit of any conventional transmission apparatus using either the digital transmission system or the amplitude modulation system. This is because, when operating either the digital transmission system or the amplitude modulation system, an extremely high linearity is demanded for amplifiers in order to minimize distorted modulation.
FIGS. 1 and 2 respectively designate schematic block diagram of power amplifying circuits executing the conventional "class AB" amplification. Referring now to FIG. 1, base of an NPN transistor 31 is connected to anode of a diode 33 via a base-side transmission line 32 containing inductance component. On the other hand, cathode of the diode 33 is connected to ground voltage GND. Emitter of the NPN transistor 31 is also connected to the ground voltage GND, whereas collector of the NPN transistor 31 is connected to power voltage V.sub.CC via a collector-side transmission line 34 containing inductance component. The power voltage V.sub.CC is delivered to the anode of the diode 33 via a resistor 15.
The power amplifying circuit shown in FIG. 1 is of such a structure which leads the power voltage delivered to the base of the NPN transistor 31 to the threshold value of the diode 33. The NPN transistor 31 activates the "class AB" amplifying operation in response to a bias voltage generated by the delivery of the power voltage to the diode 33. In order to conveniently feed the power voltage V.sub.CC to a variety of component elements, FIG. 2 designates a pair of points available for supplying the power voltage V.sub.CC, where the circuit shown in FIG. 2 has the construction identical to that is shown in FIG. 1.
On the other hand, conventionally, frequency modulation (FM) system is introduced to the analogue transmission system which is typically represented by the automobile telephone system, where the "class C" amplification mode is mainly made available.
Referring now to FIG. 3, base terminal of an NPN transistor 51 is connected to ground voltage GND via a base-side transmission line 52 containing inductance component. On the other hand, collector terminal of the NPN transistor 51 is connected to the power voltage V.sub.CC via a collector-side transmission line 53 containing inductance component.
Since the FM system is introduced to the circuit shown in FIG. 3, unlike the case of applying either the "class A" or the "class AB" amplification, there is no need of precisely executing amplification with high fidelity. Preferably, power efficiency is more important. Due to the presence of the inductance component in the base-side transmission line 52, the base terminal of the NPN transistor 51 is biased by applying a voltage below 0V. In this way, conventionally, such an amplifying circuit is made available today, which is capable of varying the biasing mode according to the introduced transmission system.
Recently, in order to deal with the increased subscribers and promote security in the execution of radio communication, mobile radio communication system including the automobile telephone unit has begun to introduce the digitally modulated transmission and reception system. Substantially, the digital radio communication system is anticipated to share the majority of the radio communication system in the future by way of replacing the conventional analogue radio communication system. Reflecting this situation, there are a wide variety of radio communication systems in this transitional period from the analogue communication system to the digital communication system. When a novel radio communication apparatus compatible with the analogue and digital communication systems is practically available, more substantial benefit can be provided for users.
Nevertheless, as mentioned above, there are a variety features in both the digital and analogue communication systems. As a result, in order to properly manage both communication systems, conventionally, two kinds of power amplifying circuits ar independently made available for materializing the "class AB" amplification and the "class C" amplification.
A hybrid integrated circuit conventionally called "hybrid IC" is mainly made available for composing the power amplifying circuit of the automobile telephone unit. Nevertheless, if two kinds of power amplifying circuits were built in an automobile telephone unit to deal with both the digital and analogue communication systems, then the telephone unit itself will take substantial dimensions. This in turn involves expensive cost, and thus, from the standpoint of economy of cost, it can hardly be materialized.